It is often desirable to measure pressure, temperature and force in environments that have very high ambient temperatures, such as at the bottom of oil or gas wells, inside nuclear reactors, or inside jet engines. Many of these environments are also characterized by inaccessibility, and the need for extreme reliability of operation. For example, the pressure and temperature at the bottom of an oil or gas well can be used to monitor the performance of the hydrocarbon reservoir. This information can aid the management of the production of that reservoir to maximise returns. The high temperatures that are often encountered at the bottom of oil or gas wells accelerate the aging and failure of electronic devices that are typically used in pressure and temperature gauge instrumentation.
As it is very costly to shut in an oil or gas well to change the downhole instrumentation, it is very important to maximize the lifetime and reliability of this instrumentation in this extreme environment. One solution is to install pressure and temperature gauges with no electronics. Strain gauge pressure sensors and platinum resistance thermometers are well known and extremely reliable at high temperatures. However, they require multiple insulated electrical conductors from the surface to the downhole sensors. It is difficult and expensive to obtain multiple conductor cable that is reliable in an oil or gas well, and the multiple pin connectors that are consequently required in the installation are also a source of high cost and potential unreliability.
One solution to measuring pressure at high temperatures, through a single conductor cable, is given in U.S. Pat. No. 4,255,973. In this sensor, a vibrating wire is connected to a bellows that is open to the fluid medium whose pressure is to be measured. Variations in pressure cause the tension in the wire to change, and hence the natural resonant frequency of this wire. The wire is placed in a magnetic field, and a current from the single conductor cable is passed through the vibrating wire. A remotely located positive feedback amplifier is connected to the other end of this single conductor cable and produces oscillations at the resonant frequency of the wire. Whilst this method admirably eliminates electronics from the sensor, it only enables one sensor to be activated per cable. Accordingly, a multiple parameter or multiple location sensor will still require several cables. In addition, the system does not appear well adapted for long distance use.